The present invention relates to packet communication devices, especially devices that interwork virtual private networks (VPNs).
On of the means of building an intranet in a company is a virtual local area network (VLAN). The IEEE 802 committee of the United States has standardized the VLAN method as IEEE 802.10.
FIG. 2 shows the VLAN packet format as defined in IEEE 802.1Q. In a VLAN 15 as defined by IEEE 802.1Q, an Internet Protocol (IP) packet 500 is transmitted as an Ethernet frame 510, in which tag control information 514 is specified. User priority 514-1, Conical Format Indicator (CFI) 514-2 and a 12-bit VLAN ID 514-3 is set in the tag control information 514. The VLAN ID is the identifier of the group that configures the VLAN. A 3-bit user priority 514-1 indicates the packet priority. The IP packet 500 also includes destination MAC address 511, source MAC address 512, Tag Protocol Identifier (TPID) 513, Internet Protocol (IP) header 501, IP Payload 502 and Frame Check Sequence (FCS) 515.
FIG. 3 shows a configuration example of a VLAN network. In FIG. 3, the network physically extends across two locations 6-1 and 3-2, like the first floor and second floor of a building. Location 6-1 contains two networks, VLAN #A (7-1-1) and VLAN #8 (7-2-1). Location 6-2 also contains two networks, VLAN #A (7-1-2) and VLAN #8 (7-2-2). VLAN #A (7-1-1) and VLAN #B (7-2-1) are multiplexed by a switching hub 2-1. Similarly, VLAN #A (7-1-2) and VLAN #B (7-2-2) are multiplexed by a switching hub 2-2. VLAN #A and VLAN #B are each assigned a unique VLAN ID. The switching hubs 2-1 and 2-2 identify the VLAN to which a packet belongs, by looking at the VLAN ID. For example, packets that VLAN #A7-1-1 transmits to location 6-2 are transferred only to VLAN #A7-1-2 by the switching hub 2-2.
On the other hand, one of the packet transfer technologies used in the Internet is multiprotocol label switching (MPLS). With MPLS, the packet transfer devices in a network perform packet transfer processing by using fixed-length connection identifiers ca led labels.
FIG. 7 shows a configuration example of an MPLS network. The transfer of a packet from terminal 4-A to terminal 4-C is explained Terminals 4-8 and 4-D are also shown as being part of the network, however no transfers to them are shown. From the destination IP address that is set in the packet, device 3-1, which is the ingress node of the MPLS network, determines the output destination of the packet and the label value to be specified in the packet. Device 3-2, which relays the packet, determines the output destination of the packet and the label value to be specified in the output packet, by using the label that was specified in the input packet. Device 3-3, which is the egress node of the MPLS network, removes the label from the packet, looks at the IP header that was set in the packet, and determines the next hop of the packet. Regarding the MPLS protocol, the Internet Engineering Task Force (IETF) is working on its standardization.
FIG. 4 shows the MPLS packet format when the Point-to-Point Protocol (PPP) 20 is used as the lower layer. In PPP-based MPLS, a four-byte shim header 522 is inserted between the PPP header 521 and the IP header 601. The shim header has a 20-bit label 521-1, a 3-bit Exp (Experimental) field 521-2, a 1-bit S bit 521-3, and an 8-bit Time to Live (TTL) field 521-4. The IETF is studying whether the Exp field 521-2 should be used as a Quality of Service (QoS) class. The MPLS packet format also includes an IF Payload 602 and a FCS 523.
FIG. 5 shows the MPLS packet format when the Asynchronous Transfer Mode (ATM) is used as the lower layer. A padding 535-1 and a trailer 535-2 are added to the IP packet 600 to form the AAL5 frame 535 (null encapsulation defined according to RFC2684 of IETF). The AAL5 frame 535 is divided into 48-byte sections, and the individual sections are each given a cell header 531 and payload 532 and become ATM cells 530-1 to 630-n. 
FIG. 6 shows the ATM cell format. In ATM-based MPLS, the set values for the virtual path identifier (VPI) 531-2 and virtual channel identifier (VCI) 531-3 in the cell header 531 are used as the label. Also, the one-bit cell loss priority (CLP) bit 531-5 10 indicates the cell discard priority. The ATM cell format also includes a Generic Flow Control (GFC) 531-1, Payload Time Identifier (PTI) 631-4 and Header Error Code (HEC) 531-6.
Hereinafter, either the shim header or the ATM cell header in which the label value is set is called the MPLS header.
In the past, communication between physically separated offices in the same company was generally carried out through leased lines. In recent years, however, the number of users who connect offices by building virtual private networks (VPNs) that use the Internet is increasing.
When VLANs located in physically separated places are connected through an MPLS network, the device positioned at the ingress rode of the MPLS network deletes the tag control information. Consequently, the information (VLAN ID) that indicates the VLAN to which the packet belongs is lost from the packet to be transferred.
Accordingly, there is a problem in which the device positioned at the of the MPLS network cannot set a VLAN ID for the packet to be output.
Furthermore, because the tag control information is deleted from the packet, the priority information (user priority) of the packet is lost. Consequently, there is a problem in which the VLAN on the side that receives the packet cannot perform the same QoS control as the VLAN on the side that sends the packet.